The present invention relates generally to soluble core processes for forming hollow chambers and passages within die-cast structures. More particularly, the present invention relates to improved salt-based soluble core processes for use with die-cast metal and/or metal matrix composite structures.
Die casting is a well-known forming technique for producing structures of various shape by pouring a liquid casting material into a pre-shaped mold or die and solidifying the liquid to form an article with the desired shape. This technique, however, does not readily lend itself to producing shapes having internal hollow cavities because the fluidity of the liquid tends to fill all open spaces within the die.
One way to produce an internal cavity in a die-cast structure is to manufacture the structure as two separate halves having respective mating flange portions and respective correlating concave portions. The flange portions are joined together by, for example, welding, and the two concave portions combine to produce an internal cavity. Such a technique, however, is limited to producing shapes having only simple cavity structures, and complex internal passages are generally precluded because of the difficulty in joining internal flange portions. Also, the mechanical properties of structures made by such a technique are likely to be limited by the mechanical properties at the joint region, and thus may be limited by the joining technique used. Further, not all materials can be easily joined.
Soluble core processes have emerged as an attractive alternative method for producing internal hollow cavities and passages in die-cast structures. In a typical soluble core process, a solid core having the dimensions of a desired internal cavity is produced by die casting, as described above. The core may include arm portions that are later used in removing the core. The core is positioned within a die of the desired structure, and a liquid material is cast around the core and solidified. The core is then removed by dissolving it in an appropriate solvent and/or flushing it away with an appropriate fluid, leaving a remaining structure that has a hollow core-shaped internal cavity.
Sand casting is one type of soluble core process. In this process, sand is used as the core material, and the sand is held together with binders to form the core. Once the desired structure is cast around the core, the binder holding the core together is removed by dissolving it and flushing it away with a solvent. The sand, in turn, is also flushed away with the solvent, leaving behind a structure with a hollow internal cavity. A major concern in using this process relates to the environmental hazards of the binder and the difficulty in recovering or reclaiming the binder from the solvent for reuse.
Foam casting is another type of soluble core process, in which the soluble core material is a foam. This process suffers from a number of problems, including the environmental hazards of the foam, the inability to produce a good surface finish, the inability to achieve tight tolerances, and the production of unwanted carbon deposits caused by the trapping of loose foam particles in the liquid casting which then turn into hard carbon deposits.
In contrast to the above-described soluble core processes, salt casting is a relatively environmentally friendly soluble core process capable of producing superior as-cast surface finishes. Salt casting uses a specialized casting salt that contains a high content of soda ash as the core material. The core is produced by die casting, as described above, and the core is later removed with hot water or steam under high pressure. A particular advantage of salt casting is that the salt solution is reclaimable by evaporating the water so that the salt may be reused.
However, conventional salt casting still has a number of drawbacks. One concern in salt casting is the high corrosivity of the molten salt used in die casting the core. This requires the use of special corrosion resistant furnace liners, die liners, and handling equipment. Another concern is the low thermal conductivity of the salt, which can result in non-uniform cooling of the core. If cooling occurs too rapidly, an outer shell solidifies first, and this thermally insulating outer shell deters the molten interior from cooling and solidifying. As a result, if the die is opened before the core is completely solidified, the core is likely to explode. Therefore, great efforts are expended to heat the die to prevent the core from cooling too quickly and forming an insulating shell. Yet another drawback is the need to keep salt cores at temperatures of approximately 315.degree. C. to maintain maximum strength and avoid premature fracture during subsequent casting. Still another drawback is the presence of internal porous regions in the core caused by gases emanating from the molten salt. Such porosity can result in weakening and eventual collapse of a core region during metal casting. A further drawback is the weakness of the salt core at aluminum casting temperatures. If the salt core is allowed to attain such high temperatures for extended periods of time, the core may soften and even liquefy, thus destroying the core and the aluminum structure. The possibility of softening of the core prevents conventional salt casting from being a reliable process for materials having high casting temperatures.